Organic electroluminescence device

ABSTRACT

An organic electroluminescent device ( 1 ) including an anode ( 2 ), a cathode ( 6 ), and at least a first layer ( 3 ), a second layer ( 4 ), and a third layer ( 5 ) provided between the anode ( 2 ) and the cathode ( 6 ) in that order from the anode side. At least one of the first to third layers ( 3 ), ( 4 ), and ( 5 ) includes a phosphorescent compound. At least one of the first to third layers ( 3 ), ( 4 ), and ( 5 ) is an emitting layer. At least three compounds respectively forming the first layer ( 3 ), the second layer ( 4 ), and the third layer ( 5 ) other than the phosphorescent compound are compounds of the following formula ( 1 ). 
     
       
         
         
             
             
         
       
     
     wherein R 1  to R 7  each represent a hydrogen atom or a substituent, provided that adjacent substituents may form a ring.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an organic electroluminescent device. Inparticular, the invention relates to a phosphorescent organicelectroluminescent device.

2. Description of Related Art

An organic electroluminescent device (organic EL device) using anorganic material is a promising solid-state emitting type inexpensiveand large full-color display device, and has been extensively developed.

In general, when an electric field is applied between electrodes of anorganic EL device, electrons are injected from the cathode and holes areinjected from the anode. The electrons recombine with the holes in theemitting layer to produce an excited state, and the energy is emitted aslight when the excited state returns to the ground state.

An organic EL device includes an emitting layer, a pair of opposingelectrodes positioned on either side of the emitting layer, and a layerwhich transports holes or electrons to the emitting layer. As a specificexample of the configuration of the organic EL device, a configurationincluding anode/hole transporting layer/emitting layer/electrontransporting layer/cathode has been known. The hole transporting layerassists in transporting holes generated from the anode to the emittinglayer, and the electron transporting layer assists in transportingelectrons generated from the cathode to the emitting layer.

Patent document 1 discloses use of a compound having a carbazole groupas a host material for the hole transporting layer (electrontransporting layer) and the emitting layer. The patent document 1utilizes a non-conjugated polymer compound for the hole transportinglayer to obtain a device with a green emission of 371 m/W. However, thecompound of the patent document 1 does not achieve a satisfactoryluminous efficiency.

Patent document 2 utilizes a compound having a carbazole group for thehole transporting layer (electron transporting layer). Patent document 3utilizes a compound having a carbazole group for the hole transportinglayer and the emitting layer.

-   [Patent document 1] JP-A-2004-220931-   [Patent document 2] JP-A-2002-203683-   [Patent document 3] U.S. Pat. No. 6,863,997

An object of the invention is to provide a low-voltage, high-efficiency,and long-lived organic EL device.

SUMMARY OF THE INVENTION

According to the invention, the following organic EL device is provided.

1. An organic electroluminescent device comprising an anode, a cathode,and at least a first layer, a second layer, and a third layer providedbetween the anode and the cathode in that order from the anode side, atleast one of the first to third layers including a phosphorescentcompound, at least one of the first to third layers being an emittinglayer, and at least three compounds respectively forming the firstlayer, the second layer, and the third layer other than thephosphorescent compound being compounds of the following formula (1),

wherein R¹ to R⁷ each represent a hydrogen atom or a substituent,provided that adjacent substituents may form a ring.2. The organic electroluminescent device according to 1, wherein thecompound of the formula (1) is a compound of the following formula (2),

wherein R¹ to R⁵ each represent a hydrogen atom or a substituent, and Arepresents a substituted or unsubstituted six to eight-memberedaliphatic ring or aromatic ring which may contain a nitrogen atom,provided that adjacent substituents may form a ring.3. The organic electroluminescent device according to 1 or 2, whereinthe compound of the formula (1) or (2) is a compound of the followingformula (3) or (4),

wherein R¹ to R⁵ or R⁸ to R¹¹ each represent a hydrogen atom or asubstituent, provided that adjacent substituents may form a ring.4. The organic electroluminescent device according to 3, wherein thecompound forming the first layer and the compound forming the secondlayer are compounds of the formula (3) in which at least one of R¹ to R⁵and R⁸ to R¹¹ is a substituent having an aromatic skeleton.5. The organic electroluminescent device according to 4, wherein thecompound forming the first layer is a compound of the formula (3) inwhich at least one of R¹ to R⁵ and R⁸ to R¹¹ has an aromaticgroup-substituted amino skeleton, and the compound forming the thirdlayer is a compound of the formula (3) in which at least one of R¹ to R⁵and R⁸ to R¹¹ has a nitrogen-containing aromatic five-membered ring, anitrogen-containing aromatic six-membered ring, or a condensed ring ofthese rings.6. The organic electroluminescent device according to 5, wherein thecompound forming the third layer is a compound of the formula (3) inwhich at least one of R¹ to R⁵ and R⁸ to R¹¹ has a nitrogen-containingaromatic six-membered ring.7. The organic electroluminescent device according to any one of 1 to 6,wherein the compounds of the formulas (1) to (4) are compounds which donot have a molecular weight distribution.8. The organic electroluminescent device according to any one of 1 to 7,wherein at least two of the three compounds have a singlet energy levelof 3.3 eV or more.9. The organic electroluminescent device according to any one of 1 to 8,wherein at least two of the three compounds have a lowest triplet energylevel of 2.7 eV or more.10. The organic electroluminescent device according to any one of 1 to9, wherein the phosphorescent compound has a lowest triplet energy levelof 2.5 eV or more.11. The organic electroluminescent device according to any one of 1 to10, wherein the second layer is an emitting layer, and the first layerand the third layer contact the emitting layer.

According to the invention, an organic EL device can be provided whichis driven at a low voltage and exhibits a high efficiency and a longlifetime.

In more detail, since different interactions between the organic layerscan be reduced by introducing a common molecular group into the firstlayer, the emitting layer, and the second layer, a carrier mobilitybarrier between each organic layer can be reduced. Therefore, the devicecan be driven at a significantly reduced voltage, whereby a highlyefficient device can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing an embodiment of an organic ELdevice according to the invention.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENT

An organic EL device according to the invention includes an anode, acathode, and at least a first layer, a second layer, and a third layerprovided between the anode and the cathode in that order from the anodeside.

FIG. 1 is a view showing a configuration of an organic EL deviceaccording to one embodiment of the invention.

As shown in FIG. 1, an organic EL device 1 has a structure in which ananode 2, a first layer 3, a second layer 4, a third layer 5, and acathode 6 are stacked.

At least one of the first layer 3, the second layer 4, and the thirdlayer 5 includes a phosphorescent compound.

At least one of the first layer 3, the second layer 4, and the thirdlayer 5 is an emitting layer. For example, one of the first layer 3, thesecond layer 4, and the third layer 5 may be an emitting layer. It ispreferable that the second layer 4 be an emitting layer, and theemitting layer include a phosphorescent compound. Two or more of thefirst layer 3, the second layer 4, and the third layer 5 may be emittinglayers. When two or more emitting layers are provided, it is preferablethat at least one emitting layer include a phosphorescent compound.

As shown in FIG. 1, the first layer 3 and the third layer 5 preferablycontact the second layer 4. Note that an intermediate layer may beprovided between the first layer 3 and the second layer 4 or between thethird layer 5 and the second layer 4. An intermediate layer may also beprovided between the anode 2 and the first layer 3 or between thecathode 6 and the third layer 5.

When the first layer 3, the second layer 4, or the third layer 5 is notan emitting layer, the first layer 3 and/or the second layer 4 on theanode 2 side may be a layer having a hole transporting property such asa hole injecting layer or a hole transporting layer, and the secondlayer 4 and/or the third layer 5 on the cathode 6 side may be a layerhaving an electron transporting property such as an electron injectinglayer or electron transporting layer.

The hole transporting layer, electron transporting layer, and emittinglayer may include a phosphorescent compound.

In the invention, at least three compounds respectively forming thefirst layer, the second layer, and the third layer other than aphosphorescent compound are compounds of the following formula (1).

wherein R¹ to R⁷ each represent a hydrogen atom or a substituent,provided that adjacent substituents may form a ring.

R⁶ and R⁷ form a ring to produce a compound of the following formula(2).

wherein R¹ to R⁵ each represent a hydrogen atom or a substituent, and Arepresents a substituted or unsubstituted six to eight-memberedaliphatic ring or aromatic ring which may contain a nitrogen atom,provided that adjacent substituents may form a ring.

A compound of the following formula (3) or (4) is preferable.

wherein R¹ to R⁵ and R⁸ to R¹¹ each represent a hydrogen atom or asubstituent, provided that adjacent substituents may form a ring.

In the formulas (2) to (4), R¹ to R⁴ and R⁸ to R¹¹ are preferably ahydrogen atom, an alkyl group having 1 to 10 carbon atoms, an alkoxygroup having 1 to 10 carbon atoms, a cyano group, an aryl group having 6to 12 carbon atoms, or a halogenated alkyl group having 1 to 10 carbonatoms, and more preferably a hydrogen atom, and R⁵ is a group having anitrogen-containing aromatic skeleton. R⁵ is preferably a group havingan aromatic group-substituted tertiary amino skeleton, carbazole group,triazine ring and/or pyrimidine ring, and more preferably a group havingan aromatic group-substituted tertiary amino skeleton, carbazole group,and/or pyrimidine ring.

The compound forming the first layer is preferably a compound of theformula (3) in which at least one of R¹ to R⁵ and R⁸ to R¹¹ has anitrogen-containing aromatic skeleton.

The nitrogen-containing aromatic skeleton includes an aromaticgroup-substituted amino skeleton, a nitrogen-containing aromatic ringskeleton, and the like. The nitrogen-containing aromatic ring skeletonincludes a nitrogen-containing aromatic five-membered ring, anitrogen-containing aromatic six-membered ring, and a condensed ring ofthese rings.

Examples of the aromatic group-substituted amino skeleton and thenitrogen-containing aromatic six-membered ring are given below.

The compound forming the first layer is preferably a compound of theformula (3) in which R¹ to R⁴ and R⁸ to R¹¹ are a hydrogen atom, analkyl group having 1 to 10 carbon atoms, an alkoxy group having 1 to 10carbon atoms, a cyano group, an aryl group having 6 to 12 carbon atoms,or a halogenated alkyl group having 1 to 10 carbon atoms, and morepreferably a hydrogen atom, and R⁵ has a carbazole group and an aromaticgroup-substituted tertiary amino skeleton.

The compound forming the third layer is preferably a compound of theformula (3) in which at least one of R¹ to R⁵ and R⁸ to R¹¹ has anitrogen-containing aromatic skeleton.

As described above, the nitrogen-containing aromatic ring skeletonincludes a nitrogen-containing aromatic five-membered ring, anitrogen-containing aromatic six-membered ring, and a condensed ring ofthese rings, and is preferably a nitrogen-containing aromaticsix-membered ring.

The compound forming the third layer is preferably a compound of theformula (3) in which R¹ to R⁴ and R⁸ to R¹¹ are a hydrogen atom, analkyl group having 1 to 10 carbon atoms, an alkoxy group having 1 to 10carbon atoms, a cyano group, an aryl group having 6 to 12 carbon atoms,or a halogenated alkyl group having 1 to 10 carbon atoms, and morepreferably a hydrogen atom, and R⁵ has a pyrimidine ring, a pyridinering, a triazine ring, or a nitrogen-containing aromatic condensed fiveor six-membered ring, and preferably a pyrimidine ring.

The compound forming the second layer is preferably a compound of theformula (3) in which at least one of R¹ to R⁵ and R⁸ to R¹¹ has anitrogen-containing aromatic skeleton.

The nitrogen-containing aromatic ring skeleton includes anitrogen-containing aromatic five-membered ring, a nitrogen-containingaromatic six-membered ring, and a condensed ring of these rings, and ispreferably a condensed ring.

The compound forming the second layer is preferably a compound of theformula (3) in which R¹ to R⁴ and R⁸ to R¹¹ are a hydrogen atom, analkyl group having 1 to 10 carbon atoms, an alkoxy group having 1 to 10carbon atoms, a cyano group, an aryl group having 6 to 12 carbon atoms,or a halogenated alkyl group having 1 to 10 carbon atoms, and morepreferably a hydrogen atom, and R⁵ has a carbazole group, arylene group,or nitrogen-containing hetero ring, and preferably a carbazole group.

The compounds respectively forming the first and second layers or thefirst to third layers are preferably compounds of the formula (3) inwhich at least one of R¹ to R⁵ and R⁸ to R¹¹ has an aromatic skeleton.Particularly preferably, at least one of R¹ to R⁵ and R⁸ to R¹¹ has anitrogen-containing aromatic skeleton.

It is preferable that the compound forming the first layer be a compoundof the formula (3) in which at least one of R¹ to R⁵ and R⁸ to R¹¹ hasan aromatic group-substituted amino skeleton, and the compound formingthe third layer be a compound of the formula (3) in which at least oneof R¹ to R⁵ and R⁸ to has a nitrogen-containing aromatic five-memberedring, a nitrogen-containing aromatic six-membered ring, or a condensedring of these rings. It is particularly preferable that at least one ofR¹ to R⁵ and R⁸ to R¹¹ of the compound forming the third layer has anitrogen-containing aromatic six-membered ring.

It is more preferable that the three compounds forming the first layer,the second layer, and the third layer be compounds of the formula (3) inwhich R¹ to R⁴ and R⁸ to R¹¹ are a hydrogen atom, an alkyl group having1 to 10 carbon atoms, an alkoxy group having 1 to 10 carbon atoms, acyano group, an aryl group having 6 to 12 carbon atoms, or a halogenatedalkyl group having 1 to 10 carbon atoms, and preferably a hydrogen atom,and R⁵ has a nitrogen-containing aromatic skeleton. R⁵ is preferably agroup having an aromatic group-substituted amino skeleton, a carbazolegroup, and/or a pyrimidine ring. Specific preferred examples of thecompound forming the first layer, the compound forming the second layer,and the compound forming the third layer have been described above.

It is preferable that the compounds of the formulas (1) to (4) becompounds which do not have a molecular weight distribution.

It is preferable that at least two of the compounds respectively formingthe first layer, the second layer, and the third layer other than aphosphorescent compound have a singlet energy level of 3.3 eV or more,and preferably 3.4 eV or more. In this case, a blue phosphorescentdevice can be more efficiently caused to emit light.

It is preferable that at least two of the compounds respectively formingthe first layer, the second layer, and the third layer other than aphosphorescent compound have a lowest triplet energy level of 2.7 eV ormore, and preferably 2.8 eV or more. In this case, a blue phosphorescentdevice can be more efficiently caused to emit light.

It is preferable that the phosphorescent compound have a lowestexcitation energy level of 2.5 eV or more, and preferably 2.6 eV ormore. In this case, blue light with higher energy can be efficientlyoutcoupled.

The organic EL device according to the invention has a device structurein which three or more layers are stacked between the electrodes. Thefollowing structures can be given as examples of the device structure.

1. Anode, hole transporting layer, electron blocking layer, emittinglayer, electron transporting layer, and cathode2. Anode, hole transporting layer, emitting layer, hole blocking layer,electron transporting layer, and cathode3. Anode, hole injecting layer, hole transporting layer, emitting layer,electron transporting layer, electron injecting layer, and cathode4. Anode, hole transporting layer, electron blocking layer, emittinglayer, electron transporting layer, electron injecting layer, andcathode5. Anode, hole transporting layer, emitting layer, emitting layer, andcathode6. Anode, emitting layer, emitting layer, electron transporting layer,and cathode7. Anode, hole transporting layer, emitting layer, emitting layer,electron transporting layer, and cathode8. Anode, hole transporting layer, electron blocking layer, emittinglayer, and cathode9. Anode, emitting layer, electron transporting layer, electroninjecting layer, and cathode

Each layer of the organic EL device according to the invention isdescribed below in more detail.

The emitting layer has a function of allowing injection of holes fromthe anode or the hole injecting layer upon application of an electricfield, a function of allowing injection of electrons from the cathode orthe electron injecting layer, a function of allowing the injectedcharges (electrons and holes) to move due to the force of the electricfield, and a function of allowing the electrons and the holes torecombine to emit light. The emitting layer of the organic EL deviceaccording to the invention preferably includes a phosphorescent compoundand a host compound of which the guest compound is the phosphorescentcompound.

The phosphorescent compound is not particularly limited insofar as thephosphorescent compound emits phosphorescence in the temperature rangein which the device operates. It is preferable to select a compound witha lowest triplet energy level of 2.5 eV or more. As specific examples ofsuch a compound, metal complexes such as Ir, Pt, Os, Pd, and Aucomplexes can be given. Of these, Ir and Pt complexes are preferable.Specific examples are given below.

wherein Me indicates a methyl group.

As examples of the host compound of the formula (1), a compound having asubstituted or unsubstituted indole group, a compound having asubstituted or unsubstituted carbazole group, a compound having asubstituted or unsubstituted azacarbazole group, and the like can begiven. Specific examples of the host compound are given below.

It is preferable that the T₁ level (energy level in the lowest tripletexcited state) of the host compound be greater than the T₁ level of theguest compound.

The emitting layer is formed by codepositing the host compound and thephosphorescent compound, for example. This allows formation of anemitting layer in which the phosphorescent compound is doped with thehost compound.

The hole injecting layer and the hole transporting layer are not limitedinsofar as these layers have one of a function of injecting holes fromthe anode, a function of transporting holes, and a function of blockingelectrons injected from the cathode.

As specific examples of the compound of the formula (1) forming the holeinjecting layer and the hole transporting layer, a compound having asubstituted or unsubstituted indole skeleton, a compound having asubstituted or unsubstituted carbazole skeleton, a compound having asubstituted or unsubstituted azacarbazole skeleton, and the like can begiven. As specific examples of the nitrogen-containing aromatic skeletonincluded in a preferred substituent, a carbazole skeleton, triazoleskeleton, pyrazole skeleton, oxazole skeleton, oxadiazole skeleton,quinoxaline skeleton, imidazole skeleton, molecular skeleton in whichthese skeletons are condensed, phenylenediamine skeleton, arylamineskeleton, amino-substituted chalcone skeleton, aromatic tertiary amineskeleton, styrylamine skeleton, and the like can be given. Theseskeletons may be substituted or unsubstituted.

Specific examples of the hole transporting compound are given below.

The hole injecting layer and the hole transporting layer may have asingle-layer structure formed of only a layer of one, or two or morecompounds selected from the above compounds, or may have a stackedstructure including a layer of one, or two or more compounds selectedfrom the above compounds.

The electron injecting layer and the electron transporting layer are notlimited insofar as these layers have a function of injecting electronsfrom the cathode and a function of transporting electrons.

As specific examples of the compound of the formula (1) forming theelectron injecting layer and the electron transporting layer, a compoundhaving a substituted or unsubstituted indole skeleton, a compound havinga substituted or unsubstituted carbazole skeleton, a compound having asubstituted or unsubstituted azacarbazole skeleton, and the like can begiven. As specific examples of the nitrogen-containing aromatic skeletonincluded in a preferred substituent, a pyridine skeleton, pyrimidineskeleton, pyrazine skeleton, triazine skeleton, triazole skeleton,oxadiazole skeleton, pyrazole skeleton, imidazole skeleton, carbazoleskeleton, indole skeleton, azacarbazole skeleton, quinoxaline skeleton,pyrrole skeleton, molecular skeletons in which these skeletons arecondensed, such as a benzimidazole skeleton and imidazopyridineskeleton, and the like can be given. Of these, a pyridine skeleton,pyrimidine skeleton, pyrazine skeleton, triazine skeleton, carbazoleskeleton, indole skeleton, azacarbazole skeleton, and quinoxalineskeleton are preferable. The above skeletons may be substituted orunsubstituted.

Specific examples of the electron transporting compound are given below.

The electron injecting layer and the electron transporting layer mayhave a single-layer structure formed of one, or two or more of the abovematerials, or may have a multilayer structure formed of a plurality oflayers of the same composition or different compositions.

An electron deficient nitrogen-containing heterocyclic group ispreferable.

In the organic EL device according to the invention, it is preferable touse an insulator or semiconductor inorganic compound as the materialforming the electron injecting/transporting layer. If the electroninjecting/transporting layer is formed of an insulator or asemiconductor, the electron injecting properties can be improved byeffectively preventing leakage of current. As such an insulator, it ispreferable to use at least one metal compound selected from the groupconsisting of an alkali metal chalcogenide, alkaline earth metalchalcogenide, alkali metal halide, and alkaline earth metal halide. Ifthe electron injecting/transporting layer is formed of an alkali metalchalcogenide or the like, the electron injecting properties can befurther improved.

As examples of preferred alkali metal chalcogenides, Li₂O, Na₂S, Na₂Se,and Na₂O can be given. As examples of preferred alkaline earth metalchalcogenides, CaO, BaO, SrO, BeO, BaS, and CaSe can be given. Asexamples of preferred alkali metal halides, LiF, NaF, KF, LiCl, KCl,NaCl, and the like can be given. As examples of preferred alkaline earthmetal halides, fluorides such as CaF₂, BaF₂, SrF₂, MgF₂, and BeF₂ andhalides other than the fluorides can be given.

As examples of the semiconductor forming the electron injecting layerand the electron transporting layer, a single material or a combinationof two or more of an oxide, nitride, or oxynitride containing at leastone element selected from Ba, Ca, Sr, Yb, Al, Ga, In, Li, Na, Cd, Mg,Si, Ta, Sb, and Zn, and the like can be given. It is preferable that theinorganic compound forming the electron transporting layer be amicrocrystalline or amorphous insulating thin film. If the electrontransporting layer is formed of such an insulating thin film, a moreuniform thin film is formed, whereby pixel defects such as dark spotscan be reduced. As examples of such an inorganic compound, theabove-mentioned alkali metal chalcogenide, alkaline earth metalchalcogenide, alkali metal halide, and alkaline earth metal halide canbe given.

In the organic EL device according to the invention, the electroninjecting layer and/or the electron transporting layer may include areductive dopant with a work function of 2.9 eV or less. In theinvention, the reductive dopant is a compound which increases electroninjecting efficiency.

In the invention, it is preferable that the reductive dopant be added tothe interfacial region between the cathode and the organic thin filmlayer so that the reductive dopant reduces the organic layer containedin the interfacial region to produce anions. A preferred reductivedopant is at least one compound selected from the group consisting of analkali metal, alkaline earth metal oxide, alkaline earth metal, rareearth metal, alkali metal oxide, alkali metal halide, alkaline earthmetal oxide, alkaline earth metal halide, rare earth metal oxide orhalide, alkali metal complex, alkaline earth metal complex, and rareearth metal complex.

As examples of preferred reductive dopants, at least one alkali metalselected from the group consisting of Na (work function: 2.36 eV), K(work function: 2.28 eV), Rb (work function: 2.16 eV), and Cs (workfunction: 1.95 eV), and at least one alkaline earth metal selected fromthe group consisting of Ca (work function: 2.9 eV), Sr (work function:2.0 to 2.5 eV), and Ba (work function: 2.52 eV) can be given. A materialwith a work function of 2.9 eV is particularly preferable. The reductivedopant is preferably at least one alkali metal selected from the groupconsisting of K, Rb, and Cs, more preferably Rb or Cs, and mostpreferably Cs. These alkali metals exhibit particularly high reducingcapability so that an increase in the luminance and the lifetime of theorganic EL device can be achieved by adding a relatively small amount ofalkali metal to the electron injection region.

As the alkaline earth metal oxide, BaO, SrO, CaO, and Ba_(x)Sr_(1-x)O(0<x<1), and Ba_(x)Ca_(1-x)O (0<x<1) as mixtures thereof are preferable.As examples of the alkali oxide or alkali fluoride, LiF, Li₂O, NaF, andthe like can be given. The alkali metal complex, alkaline earth metalcomplex, and rare earth metal complex are not particularly limitedinsofar as the complex contains at least one of an alkali metal ion,alkaline earth metal ion, and rare earth metal ion as the metal ion.

As examples of the ligand, quinolinol, benzoquinolinol, acridinol,phenanthridinol, hydroxyphenyloxazole, hydroxyphenylthiazole,hydroxydiaryloxadizole, hydroxydiarylthiadiazole, hydroxyphenylpyridine,hydroxyphenylbenzimidazole, hydroxybenzotriazole, hydroxyfurborane,bipyridyl, phenanthroline, phthalocyanine, porphyrin, cyclopentadiene,β-diketone, azomethine, derivatives thereof, and the like can be given.Note that the ligand is not limited thereto.

The reductive dopant is preferably formed in the shape of a layer orislands. The thickness of the reductive dopant is preferably 0.05 to 8nm when used in the shape of a layer.

As the method of forming the electron injecting layer and the electrontransporting layer including the reductive dopant, a method ispreferable in which an organic material which is the emitting materialor the electron injecting material which forms the interfacial region isdeposited while depositing the reductive dopant by resistance heatingdeposition to disperse the reductive dopant in the organic material. Thedispersion concentration (molar ratio) is 100:1 to 1:100, and preferably5:1 to 1:5. When forming the reductive dopant in the shape of a layer,the emitting material or the electron injecting material which is theorganic layer at the interface is formed in the shape of a layer, andthe reductive dopant is deposited alone by resistance heating depositionto a thickness of preferably 0.5 to 15 nm. When forming the reductivedopant in the shape of islands, after forming the emitting material orthe electron injecting material which is the organic layer at theinterface, the reductive dopant is deposited alone by resistance heatingdeposition to a thickness of preferably 0.05 to 1 nm.

The anode supplies holes to the hole injecting layer, the holetransporting layer, the emitting layer, and the like. It is effectivethat the anode have a work function of 4.5 eV or more. As a compound forforming the anode, a metal, alloy, metal oxide, conductive compound, amixture of these materials, or the like may be used. As specificexamples of the compound for forming the anode, conductive metal oxidessuch as tin oxide, zinc oxide, indium oxide, and tin-doped indium oxide(ITO), metals such as gold, silver, chromium, and nickel, a mixture or astacked product of the conductive metal oxide and the metal, inorganicconductive substances such as copper iodide and copper sulfide, organicconductive materials such as polyaniline, polythiophene, andpolypyrrole, a stacked product of the conductive material and ITO, andthe like can be given. Of these, the conductive metal oxide ispreferable. In particular, it is preferable to use ITO from theviewpoint of productivity, conductivity, transparency, and the like. Thethickness of the anode may be appropriately selected.

The cathode supplies electrons to the electron injecting layer, theelectron transporting layer, the emitting layer, and the like. As acompound for forming the cathode, a metal, alloy, metal halide, metaloxide, conductive compound, or a mixture of these materials may be used.As specific examples of the material for the cathode, alkali metals(e.g. Li, Na, and K) and fluorides or oxides thereof, alkaline earthmetals (e.g. Mg and Ca) and fluorides or oxides thereof, gold, silver,lead, aluminum, sodium-potassium alloy or sodium-potassium mixed metal,lithium-aluminum alloy or lithium-aluminum mixed metal, magnesium-silveralloy or magnesium-silver mixed metal, rare earth metals such as indiumand ytterbium, and the like can be given. Of these, aluminum,lithium-aluminum alloy or lithium-aluminum mixed metal, magnesium-silveralloy or magnesium-silver mixed metal, and the like are preferable. Thecathode may have a single-layer structure formed of only a layer of one,or two or more compounds selected from the above compounds, or may havea stacked structure including a layer of one, or two or more compoundsselected from the above compounds. For example, a stacked structure ofaluminum/lithium fluoride or aluminum/lithium oxide is preferable. Thethickness of the cathode may be appropriately selected.

In the organic EL device according to the invention, the formationmethod for each layer is not particularly limited. Various methods maybe utilized such as vacuum evaporation, LB method, resistance heatingdeposition, electron beam method, sputtering, molecular stack method,coating (spin coating, casting, dip coating and the like), inkjetmethod, and printing.

An organic thin film layer including a metal complex compound may beformed using a known method such as vacuum deposition, molecular beamepitaxy (MBE), or a coating method using a solution in which thematerial is dissolved in a solvent, such as dipping, spin coating,casting, bar coating, or roll coating.

In the above coating method, the metal complex compound is dissolved ina solvent to prepare a coating liquid, and the coating liquid is appliedto and dried on a desired layer (or electrode). A resin may be added tothe coating liquid. The resin may be dissolved or dispersed in thesolvent. As the resin, a non-conjugated polymer (e.g.polyvinylcarbazole) or a conjugated polymer (e.g. polyolefin polymer)may be used. As examples of the resin, polyvinyl chloride,polycarbonate, polystyrene, polymethyl methacrylate, polybutylmethacrylate, polyester, polysulfone, polyphenylene oxide,polybutadiene, poly(N-vinylcarbazole), hydrocarbon resin, ketone resin,phenoxy resin, polyamide, ethyl cellulose, vinyl acetate, ABS resin,polyurethane, melamine resin, unsaturated polyester resin, alkyd resin,epoxy resin, silicon resin, and the like can be given.

The thickness of each organic layer of the organic EL device accordingto the invention is not particularly limited. In general, defects suchas pinholes tend to occur when the thickness is too small, and a highvoltage must be applied when the thickness is too great, resulting inpoor efficiency. Therefore, the thickness of each organic layer ispreferably several nanometers to 1 μm.

EXAMPLES

Compounds of the following formulas were used in the examples andcomparative examples. The characteristics of these compounds weremeasured using the following methods. The results are shown in Table 1.

wherein Me indicates methyl, and Ph indicates phenyl.

(1) Ionization Potential

A thin film of each material was formed, and the ionization potential ofthe thin film was measured using an “AC-1” manufactured by Riken KeikiCo., Ltd.

A glass substrate was subjected to ultrasonic cleaning for five minutesin isopropyl alcohol, five minutes in water, and five minutes inisopropyl alcohol, and then subjected to UV cleaning for 30 minutes. Afilm of a measurement target material was formed on the glass substrateusing a vacuum deposition device. The film was formed to a thickness of2000 angstroms using an “SGC-8MII” manufactured by Showa Shinku Co.,Ltd. at a final vacuum of 5.3×10⁻⁴ Pa or less and a deposition rate of 2angstroms/sec.

The ionization potential was measured using an atmospheric photoelectronspectrometer (“AC-1” manufactured by Riken Keiki Co., Ltd.). Lightobtained by dispersing ultraviolet rays from a deuterium lamp using aspectroscope was applied to the thin film sample, and the emittedphotoelectrons were measured using an open counter. The intersection ofthe background and the square root of the quantum yield in thephotoelectron spectrum in which the square root of the quantum yield wasplotted along the vertical axis and the energy of applied light wasplotted along the horizontal axis was taken as the ionization potential.

(2) Singlet Energy Level

The compound was dissolved in toluene to obtain a 10⁻⁵ mol/l solution.The absorption spectrum was measured using a spectro-photometer (“U3410”manufactured by Hitachi, Ltd.). A line tangent to the UV absorptionspectrum was drawn at the rising edge on the longer wavelength side, andthe wavelength (absorption edge) at which the tangent line intersectsthe horizontal axis was determined. This wavelength was converted intoan energy value to determine the energy level.

(3) Triplet Energy Level

The lowest triplet energy level T₁ was measured as follows. The lowesttriplet energy level T₁ was measured using a Fluorolog II manufacturedby SPEX at a concentration of 10 micromol/l and a temperature of 77Kusing EPA (diethyl ether:isopentane:isopropyl alcohol=5:5:2 (volumeratio)) as a solvent utilizing a quartz cell. A line tangent to theresulting phosphorescence spectrum was drawn at the rising edge on theshorter wavelength side, and the wavelength (emission edge) at which thetangent line intersects the horizontal axis was determined. Thiswavelength was converted into an energy value.

TABLE 1 Ionization Singlet Lowest triplet potential energy level energylevel Compound (eV) (eV) (eV) TCTA 5.8 3.3 2.9 Compound (A) 6.0 3.6 2.9Compound (B) 5.7 — 2.6 Compound (C) 6.0 3.9 2.9 Compound (D) 5.8 — 2.7Compound (E) 5.3 — 2.4 Alq₃ 5.8 2.7 — HMTPD 5.6 3.3 2.6 NPD 5.5 3.0 2.4Compound (F) 7.1 4.3 3.5 Compound (G) 5.8 3.3 2.9

Example 1

A glass substrate with an ITO transparent electrode (25 mm×75 mm×0.7 mm)was subjected to ultrasonic cleaning in isopropyl alcohol for fiveminutes and then subjected to UV ozone cleaning for 30 minutes. Thecleaned glass substrate with the transparent electrode was installed ina substrate holder of a vacuum deposition device, and a TCTA film with athickness of 95 nm was formed on the surface of the glass substrate onwhich the transparent electrode was formed so that the transparentelectrode was covered with the TCTA film. The TCTA film functions as ahole transporting layer. The compound (A) was deposited on the TCTA filmas a host compound to a thickness of 30 nm to form an emitting layer.The Ir metal complex compound (B) was added as a phosphorescent Ir metalcomplex dopant. The concentration of the metal complex compound (B) inthe emitting layer was adjusted to 7.5 wt %. This film functions as anemitting layer. The compound (C) was formed on this film to a thicknessof 25 nm. This film functions as an electron transporting layer. An Alq₃film was formed on this film to a thickness of 5 nm. This film functionsas an electron transporting layer. Lithium fluoride was then depositedto a thickness of 0.1 nm, and aluminum was deposited to a thickness of150 nm. This Al/LiF film functions as a cathode. An organic EL devicewas fabricated.

After sealing the resulting device, electricity was supplied to thedevice. Blue green light with a luminance of 114 cd/m² was obtained at avoltage of 5.5 V and a current density of 0.28 mA/cm². The luminousefficiency was 41 cd/A. The device was driven at a constant current andan initial luminance of 200 cd/m². The period of time until theluminance was halved to 100 cd/m² was measured and found to be 2050hours.

Example 2

An organic EL device was fabricated in the same manner as in Example 1except for using the compound (D) instead of the compound (B). Aftersealing the resulting device, electricity was supplied to the device inthe same manner as in Example 1.

Blue green light with a luminance of 113 cd/m² was obtained at a voltageof 5.5 V and a current density of 0.35 mA/cm². The luminous efficiencywas 32 cd/A. The device was driven at a constant current and an initialluminance of 200 cd/m². The period of time until the luminance washalved to 100 cd/m² was measured and found to be 730 hours.

Example 3

An organic EL device was fabricated in the same manner as in Example 1except for using the compound (E) instead of the compound (B). Aftersealing the resulting device, electricity was supplied to the device inthe same manner as in Example 1.

Green light with a luminance of 108 cd/m² was obtained at a voltage of5.5 V and a current density of 0.14 mA/cm². The luminous efficiency was77 cd/A and 44 μm/W. The device was driven at a constant current and aninitial luminance of 1500 cd/m². The period of time until the luminancewas halved to 750 cd/m² was measured and found to be 3210 hours.

Comparative Example 1

An organic EL device was fabricated in the same manner as in Example 1except for using HMTPD instead of TCTA.

After sealing the resulting device, electricity was supplied to thedevice in the same manner as in Example 1.

Blue green light with a luminance of 106 cd/m² was obtained at a voltageof 7.4 V and a current density of 0.92 mA/cm². The luminous efficiencywas 12 cd/A. The device was driven at a constant current and an initialluminance of 200 cd/m². The period of time until the luminance washalved to 100 cd/m² was measured and found to be 298 hours.

Comparative Example 2

An organic EL device was fabricated in the same manner as in Example 1except for using NPD instead of TCTA.

After sealing the resulting device, electricity was supplied to thedevice in the same manner as in Example 1.

Blue green light with a luminance of 100 cd/m² was obtained at a voltageof 7.3 V and a current density of 1.50 mA/cm². The luminous efficiencywas 6 cd/A. The device was driven at a constant current and an initialluminance of 200 cd/m². The period of time until the luminance washalved to 100 cd/m² was measured and found to be 380 hours.

Comparative Example 3

An organic EL device was fabricated in the same manner as in Example 1except for using the compound (F) instead of the compound (A).

After sealing the resulting device, electricity was supplied to thedevice in the same manner as in Example 1.

Blue green light with a luminance of 100 cd/m² was obtained at a voltageof 8.3 V and a current density of 1.80 mA/cm². The luminous efficiencywas 6 cd/A. The device was driven at a constant current and an initialluminance of 200 cd/m². The period of time until the luminance washalved to 100 cd/m² was measured and found to be 180 hours.

Comparative Example 4

An organic EL device was fabricated in the same manner as in Example 3except for using the compound (F) instead of the compound (A).

After sealing the resulting device, electricity was supplied to thedevice in the same manner as in Example 1.

Green light with a luminance of 110 cd/m² was obtained at a voltage of6.8 V and a current density of 0.65 mA/cm². The luminous efficiency was17 cd/A. The device was driven at a constant current and an initialluminance of 1500 cd/m². The period of time until the luminance washalved to 750 cd/m² was measured and found to be 1060 hours.

Comparative Example 5

An organic EL device was fabricated in the same manner as in Example 3except for using the compound (G) instead of the compound (C).

After sealing the resulting device, electricity was supplied to thedevice in the same manner as in Example 1.

Green light with a luminance of 101 cd/m² was obtained at a voltage of6.2 V and a current density of 0.43 mA/cm². The luminous efficiency was23 cd/A. The device was driven at a constant current and an initialluminance of 1500 cd/m². The period of time until the luminance washalved to 750 cd/m² was measured and found to be 1840 hours.

TABLE 2 Hole Electron Luminous transporting transporting Voltage Currentdensity Luminance efficiency Initial luminance Half life layer Emittinglayer layer (V) (mA/cm²) (cd/m²) (cd/A) (cd/m²) (h) Example 1 TCTACompound (A) Compound (C) 5.5 0.28 114 41 200 2050 Compound (B) Alq₃Example 2 TCTA Compound (A) Compound (C) 5.5 0.35 113 32 200 730Compound (D) Alq₃ Example 3 TCTA Compound (A) Compound (C) 5.5 0.14 10877 1500 3210 (441 m/W) Compound (E) Alq₃ Comparative HMTPD Compound (A)Compound (C) 7.4 0.92 106 12 200 298 Example 1 Compound (B) Alq₃Comparative NPD Compound (A) Compound (C) 7.3 1.50 100 6 200 380 Example2 Compound (B) Alq₃ Comparative TCTA Compound (F) Compound (C) 8.3 1.80100 6 200 180 Example 3 Compound (B) Alq₃ Comparative TCTA Compound (F)Compound (C) 6.8 0.65 110 17 1500 1060 Example 4 Compound (E) Alq₃Comparative TCTA Compound (A) Compound (G) 6.2 0.43 101 23 1500 1840Example 5 Compound (E) Alq₃

As shown in Table 2, the organic EL devices of Examples 1 to 3 aredriven at a low voltage and exhibit a high luminous efficiency and along lifetime in comparison with the organic EL devices of ComparativeExamples 1 to 5.

INDUSTRIAL APPLICABILITY

As described above in detail, the organic EL device according to theinvention exhibits a high luminous efficiency and a long lifetime andcan be used as organic EL materials of various colors including blue.The organic EL device according to the invention may be applied in thefields of a display element, display, backlight, illumination lightsource, sign, signboard, interior, and the like, and is particularlysuitable as a display element for a color display.

1-11. (canceled)
 12. An organic electroluminescent device, comprising:an anode; a cathode; a first layer; a second layer; and a third layer;wherein the first layer, the second layer and the third layer arearranged between the anode and the cathode in order from the anode tothe cathode; at least one of the first to third layers comprises aphosphorescent compound; at least one of the first to third layers is anemitting layer; at least three compounds respectively forming the firstlayer, the second layer, and the third layer other than thephosphorescent compound are compounds of the following formula (2); thephosphorescent compound has a lowest triplet energy level of 2.5 eV ormore;

wherein: each of R¹ to R⁵ independently represents a hydrogen atom or asubstituent; and A represents a substituted or unsubstituted six toeight-membered aliphatic ring or aromatic ring which may contain anitrogen atom; provided that adjacent substituents may form a ring. 13.The organic electroluminescent device according to claim 12, wherein thecompound forming the third layer is a compound of the following formula(3) or (4),

wherein each of R¹ to R⁵ or R⁸ to R¹¹ each represent a hydrogen atom ora substituent, provided that adjacent substituents may form a ring. 14.The organic electroluminescent device according to claim 13, wherein theformula (3), at least one of R¹ to R⁵ or R⁸ to R¹¹ has anitrogen-containing aromatic skeleton.
 15. The organicelectroluminescent device according to claim 14, wherein thenitrogen-containing aromatic skeleton is a nitrogen-containing aromaticsix-membered ring or a condensed ring comprising the ring.
 16. Theorganic electroluminescent device according to claim 13, wherein thecompound forming the third layer is a compound of formula (3).
 17. Theorganic electroluminescent device according to claim 13, wherein thecompound forming the third layer is a compound of formula (4).